The present invention generally relates to an electrochemical cell construction. More particularly, the present invention relates to the construction of a collector assembly used for an electrochemical cell, such as an alkaline cell.
FIG. 1 shows the construction of a conventional C-sized alkaline cell 10. As shown, cell 10 includes a cylindrically-shaped can 12 having an open end and a closed end. Can 12 is preferably formed of an electrically-conductive material such that an outer cover (not shown) welded to a bottom surface 14 at the closed end of can 12, serves as an electrical contact terminal for the cell.
Cell 10 further typically includes a first electrode material 15, which may serve as the positive electrode (also known as a cathode). The first electrode material 15 may be preformed and inserted into can 12, or may be molded in place so as to contact the inner surfaces of the can 12. For an alkaline cell, first electrode material 15 will typically include MnO.sub.2. After the first electrode 15 has been provided in can 12, a separator 17 is inserted into the space defined by first electrode 15. Separator 17 is preferably a non-woven fabric. Separator 17 is provided to maintain a physical separation of the first electrode material 15 and a mixture of electrolyte and a second electrode material 20 while allowing the transport of ions between the electrode materials.
Once separator 17 is in place within the cavity defined by first electrode 15, an electrolyte is dispensed into the space defined by separator 17 along with the mixture 20 of electrolyte and a second electrode material, which may be the negative electrode (also known as the anode). The electrolyte/second electrode mixture 20 preferably includes a gelling agent. For a typical alkaline cell, mixture 20 is formed of a mixture of an aqueous KOH electrolyte and zinc, which serves as the second electrode material. Water and additional additives may also be included in mixture 20.
Once the first electrode 15, separator 17, the electrolyte, and mixture 20 have been formed inside can 12, a preassembled collector assembly 25 is inserted into the open end of can 12. Can 12 is typically slightly tapered to have a larger diameter at its open end. This taper serves to support the collector assembly in a desired orientation prior to securing it in place. After collector assembly 25 has been inserted, an outer cover 45 is placed over collector assembly 25. Collector assembly 25 and outer cover 45 are secured in place by radially squeezing and crimping the peripheral lip of collector assembly 25 and outer cover 45 within the end edge 13 of can 12. As described further below, the primary function served by collector assembly 25 is to provide for a second external electrical contact for the electrochemical cell. Additionally, collector assembly 25 must seal the open end of can 12 to prevent the electrochemical materials therein from leaking from this cell. Additionally, collector assembly 25 must exhibit sufficient strength to withstand the physical abuse to which batteries are typically exposed. Also, because electrochemical cells may produce hydrogen gas, collector assembly 25 preferably allows the internally-generated hydrogen gas to permeate therethrough to escape to the exterior of the electrochemical cell. Further, collector assembly 25 should include some form of pressure relief mechanism to relieve pressure produced internally within the cell should this pressure become excessive. Such conditions may occur when the electrochemical cell internally generates hydrogen gas at a rate that exceeds that at which the internally-generated hydrogen gas can permeate through the collector assembly to the exterior of the cell.
The collector assembly 25 shown in FIG. 1 includes a seal 30, a collector nail 40, an inner cover 44, a washer 50, and a plurality of spurs 52. Seal 30 is shown as including a central hub 32 having a hole through which collector nail 40 is inserted. Seal 30 further includes a V-shaped portion 34 that may contact an upper surface 16 of first electrode 15.
Seal 30 also includes a peripheral upstanding wall 36 that extends upward along the periphery of seal 30 in an annular fashion. Peripheral upstanding wall 36 not only serves as a seal between the interface of collector assembly 25 and can 12, but also serves as an electrical insulator for preventing an electrical short from occurring between the positive can and negative contact terminal of the cell.
Inner cover 44, which is formed of a rigid metal, is provided to increase the rigidity and supports the radial compression of collector assembly 25 thereby improving the sealing effectiveness. As shown in FIG. 1, inner cover 44 is configured to contact central hub portion 32 and peripheral upstanding wall 36. By configuring collector assembly 25 in this fashion, inner cover 44 serves to enable compression of central hub portion 32 by collector nail 40 while also supporting compression of peripheral upstanding wall 36 by the inner surface of can 12.
Outer cover 45 is typically made of a nickel-plated steel and is configured to extend from a region defined by the annular peripheral upstanding wall 36 of seal 30 and to be in electrical contact with a head portion 42 of collector nail 40. Typically, outer cover 45 is welded to head portion 42 of collector nail 40 to prevent any loss of contact. As shown in FIG. 1, when collector assembly 25 is inserted into the open end of can 12, collector nail 40 penetrates deeply within the electrolyte/second electrode mixture 20 to establish sufficient electrical contact therewith. In the example shown in FIG. 1, outer cover 45 includes a peripheral lip 47 that extends vertically upward along the circumference of outer cover 45. By forming peripheral upstanding wall 36 of seal 30 of a length greater than that of peripheral lip 47, a portion of peripheral upstanding wall 36 may be folded over peripheral lip 47 during the crimping process so as to prevent any portion of the upper edge 13 of can 12 from coming into contact with outer cover 45.
Seal 30 is preferably formed of nylon. In the configuration shown in FIG. 1, a pressure relief mechanism is provided for enabling the relief of internal pressure when such pressure becomes excessive. Further, inner cover 44 and outer cover 45 are typically provided with apertures that allow the hydrogen gas to escape to the exterior of cell 10. The mechanism shown includes an annular metal washer 50 and a plurality of spurs 52 that are provided between seal 30 and inner cover 44. The plurality of spurs 52 each include a pointed end 53 that is pressed against a thin intermediate portion 38 of seal 30. Spurs 52 are biased against the lower inner surface of inner cover 44 such that when the internal pressure of cell 10 increases and seal 30 consequently becomes deformed by pressing upward toward inner cover 44, the pointed ends 53 of spurs 52 penetrate through the thin intermediate portion 38 of seal 30 thereby rupturing seal 30 and allowing the escape of the internally-generated gas.
Although the above-described collector assembly 25 performs all the above-noted desirable functions satisfactorily, as apparent from its cross-sectional profile, this particular collector assembly occupies a significant amount of space within the interior of the cell 10. Because the interior dimensions of the electrochemical cell are generally fixed, the greater the space occupied by the collector assembly, the less space that there is available within the cell for the electrochemical materials. Consequently, a reduction in the amount of electrochemical materials that may be provided within the cell results in a shorter service life for the cell. It is therefore desirable to design a collector assembly that occupies less space within the electrochemical cell.
It should be noted that the collector assembly construction shown in FIG. 1 is but one example of a cell construction. Other collector assemblies exist that have lower profiles and hence occupy less space within the cell. However, such collector assemblies typically achieve this reduction in occupied volume at the expense of the sealing characteristics of the collector assembly or the performance and reliability of the pressure relief mechanism. It is therefore desirable to construct a collector assembly that occupies a minimal amount of space within an electrochemical cell while still maintaining adequate sealing characteristics, and a reliable pressure relief mechanism.